Variable inductance



1952 w. H. ANDERSON ETAL 2,583,133

VARIABLE INDUCTANCE Filed June 19, 1947 j LYIJ dz wmusssss; INVENTORS William Mflndersan and flalYberl (.Frey.

ATTORNEY Patented Jan. 22, 1952 VARIABLE INDUCTANCE William H. Anderson, Elkridge, and Albert C.

Frey, Baltimore, Md., assignora to Westinghouse Electric Corporation, East Pittsburgh, l'a.. a corporation of Pennsylvania Application June 19, 1947, Serial No. 755,758

This invention relates to a variable inductance tuner for use in tuning a high frequency LC circuit. and it has for an object to provide improved apparatus of this kind.

Another object is to provide an improved variable inductance tuner comprising a flexible strap wherein the inductance is varied by varying the length of the strap.

Present day variable inductance strap tuners, of which we are familiar, are exemplified in the prior art by Patent No. 2,410,222. Lawrence. Lawrence varies the inductance of his device by altering the area embraced by a loop and hence the effective inductance of the strap comprising the loop. It is stated that this device will tune an LC circuit over a frequency range of at least 205 to 260 megacycles. This is a frequency ratio of 1.27:1. Since tuning inductance is inversely proportional to frequency squared 1 417) Lawrence's variable inductance has a tuning ratio of (l.2'l)':1=1.6:1.

When it is desired to tune a parallel LC circuit comprising a capacitance which varies from, for example, 1500 M. M. F.-250 M. M. R, such as we do, in a manner which is explained more fully hereinafter, the capacity ratio is 1500/250=6:l. This means that a variable inductance to tune this circuit to parallel or anti-resonance must also have a tuning ratio of 6:1. This ratio is far beyond the range of the Lawrence device.

Accordingly. another object is to obviate the foregoing and other less apparent objections to the prior art strap type variable inductance tuners.

Another object is to provide a variable inductance strap type tuner having a relatively high tuning ratio.

These and other objects are effected by our invention as will be apparent from the following description and claims taken in accordance with the accompanying drawing, forming a part of this application, in which: I

Figure 1 is a schematic illustration of our invention adapted for tuning a high frequency dielectric heating circuit; and

Fig. 2 is a partial perspective, with parts broken away for clarity of illustration, of our variable inductance tuner unit per se.

Referring to Fig. 1 in detail, for the purpose of illustration, we show our variable strap inductance tuner being installed for tuning a high frequency dielectric heating system. The heating system comprises a source ID of high frequency oscillations which is connected by means of a transformer II and a coaxial cable I2 to a pair of spaced heating or energy utilizing electrodes II and It.

In order to obtain optimum power transfer from the coaxial cable I2 to the heating or energy utilizing electrodes I4 and I5, the circuit comprising the electrodes II and I5 should present a terminating impedance looking into the cable I2 equal to the characteristic impedance f the cable I2. To tune or adjust the circuit so as to present the proper impedance to the cable I2, we employ our improved variable strap inductance tuner I8. It is seen that the electrodes I4 and I5 comprise the two plates of a condenser, which presents a capacitive reactance to the cable I2. We connect our variable strap inductance I8 across the electrodes II and I5 to present an inductive reactance to the cable I2. The parallel LC circuit comprising the electrodes I4 and II and the variable inductance I8 may be tuned to parallel or anti-resonance by adjusting the inductance I2. It is understood that the inductance I8 may be adjusted so that the LC circuit will present any desired terminating impedance to the cable I2.

Our variable strap inductance tuner I8 is best illustrated by Fig. 2. It comprises a bracket 28 which carries a pair of rolls 2| and 22. The roll 2| has a gear 24 mounted at one end thereof. The roll 22 has a gear 25 mounted on the corresponding end thereof so that it will ordinarily mesh with the gear 24. A shaft 26 extends from the roll 22' for rotating the same. When the roll 22 is rotated, rotary motion is also imparted to the roll 2| through the meshing gears 24 and 28. A substantially U-shaped flexible metal strap member 2'! provides the variable inductance for our tuner I8. The strap 21 has one end attached to the roll 22 so that as the roll 22 is rotated in one direction the strap 21 is wound about the roll 22, as indicated at 28, and the effective length and the resulting inductance of the strap 21 is reduced. correspondingly, when the roll 22 is rotated in the opposite direction, the strap 21 unwinds from about the roll 22 and thus in-- creases its effective length and the resulting inductance. A lever 29 is attached to the roll 2| and pivoted to the bracket at 20. This lever is biased downwardly by a spring 80. This arrangement holds the roll 2| down firmly against the strap 21 and insures good tight winding of the strap 2'! and good electrical contact between the strap 21 and the roll 22. The roll 22 also makes good electrical contact with the bracket 20.

The variable strap inductor 18 may be installed in a system such as illustrated in Fig. 1 by attaching the bracket 20 to the lower heating electrode I by rivets or bolts which extend through holes 3| provided in the bracket 20 and through the lower heating electrode Hi. The other end of the strap may be attached to the upper electrode l4 by rivets or bolts which extend through a hole 32 therein and through the upper electrode l4. However, it is understood that the variable strap inductance It may be installed in a system in any other suitable manner.

When our variable strap inductor I8 is used to tune a system such as illustrated in Fig. 1, some suitable means such as a'reversible electric motor 35 is employed for adjusting the strap 21. The motor 35 is connected to drive the shaft 26 to wind or unwind the strap 21 about the roll 22. A suitable source of power 38 is provided for operating the motor 35. A field winding 31 for the motor and a variac 38 for adjusting the motor field is provided in circuit with the motor 35 and the power source 35. A first push button 40 is provided in circuit with the motor 35 and the power source 36. When this push button is operated to close its associated sets of contacts 42 and 43, the motor 35 rotates in a direction to wind the strap 21 about the roll 22 and thus reduce the length of the strap and consequently reduce its effective inductance. A second push button 45 is provided in another circuit with the motor 35 and the power source 35. When this push button is operated to close its assciated pairs of contacts 46 and 41, the motor 35 operates in a direction to unwind the strap 21 from about the roll 22 and thus increase the length of the strap and consequently increase its effective inductance. It is understood that when the push buttons 40 and 45 are not held closed they are spring biased to open circuit position.

When using our variable strap inductor l8 in a system as illustrated in Fig. 1, a charge 48 to be heated is placed between the heating electrodes l4 and I5 and the tuner 18 is adjusted until the circuit comprising the heating electrodes I4 and I5 and the strap 21 is substantially parallel or anti-resonant. This condition is observed by three standing wave meters 50, 5| and 52 connected to the coaxial line l2 and spaced apart a distance D, which is approximately wavelength of the oscillations being supplied from the source 10. When the impedance of the circuit comprising the electrodes 14 and I5 and the strap inductance 21 matches the impedance of the cable l2, the three standing wave meters will all indicate the same value of voltage. However, if the impedance of the circuit does not match the impedance of the cable l2, the meters 50, 5| and 52 will indicate different values. In such a case, the length of the strap 21 is adjusted until the standing wave meters all indicate the same value, which is an indication that the circuit comprising the electrodes l4 and I5 and the strap inductance 21 is properly terminated on the cable l2.

At the beginning of the heating operation, the capacitance of the electrodes l4 and I5 and the charge 48 is relatively large and a small amount of inductance is required to properly tune the circuit, in which instance most of the strap 21 is wound about the roll 22. However, as the charge 48 is heated. moisture is driven out of it and the dielectric constant of the charge decreases.

This decreases the capacitance of the circuit so that more inductance must be inserted to tune the circuit to present the proper impedance to the cable l2. This is accomplished by pressing the push button 45 to start the motor 35 to unwind the strap 21 from about the roll 22. In the meantime, the meters 50, 5| and 52 are being watched and as soon as they all indicate the same value of voltage the push button 45 is released to stop the motor 35 for the impedance of the circuit comprising the electrodes l4 and I5 and the strap 21 now matches the impedance of the cable [2. This is the condition for optimum power transfer. It will be obvious that the circuit comprising the electrodes 14 and I5 and the strap 21 may be tuned over the entire operating range merely by pushing the proper push button to adjust the length of the strap 21 to insert or remove the proper amount of inductance.

The variable inductance tuner It has been used with success in dielectric heating systems. In these systems with electrodes l4 and I5 having dimensions of 64 inches by 88 inches and spaced from 6 to 14 inches apart, the capacitance of the condenser provided by the electrodes 14 and I5 varied between approximately 1500 M. M. F. and 250 M. M. F. during the process of heating a charge. This is a capacity tuning ratio of 1500/250 or 6:1. This means that the variable inductance required to tune the circuit over the entire heating cycle also had a tuning ratio of 6:1. It is seen that this ratio is considerably higher than the tuning ratio anticipated by the prior art.

Although we have illustrated our variable inductance strap tuner l8 for tuning a high frequency heating circuit for optimum power transfer to the energy utilizing system or heating electrodes, it is understood that it may be used in any application where a variable inductance is required.

From the foregoing description, it is seen that we have provided an improved strap type variable inductance which is adaptable for tuning high frequency LC circuits over a substantial range.

While we have shown our invention in one form only, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit thereof, and we desire, therefore, that only such limitations shall be placed thereupon as are specifically set forth in the appended claims.

We claim as our invention:

1. A variable inductor comprising a strap formed into a single loop with one end thereof fixedly anchored, and means for varying the perimeter of said loop to vary said inductance.

2. A variable inductor comprising a strap formed into a single loop with one end thereof fixedly anchored, and means associated with one end of said strap for varying the perimeter of said loop to vary said inductance.

3. A variable inductance tuner comprising a flexible conductive strap formed into a single loop having one end thereof fixedly anchored, and means including a roll about which one end of said strap is wound for varying the length of said strap to vary said inductance.

4. Apparatus for varying the impedance of a circuit having inductance and capacitance comprising a strap formed into a single loop with one end thereof fixedly anchored and means for varying the perimeter of said loop to vary the inductance of said circuit over a range such as to produce a tuning ratio of six to one.

5. A variable inductor comprising an electrical conductor formed into a single loop with one end thereof fixedly anchored and means for varying the perimeter of said conductor to vary the inductance of said inductor.

6. In combination: a strap formed into a single loop with one end thereof fixedly anchored, means for varying the perimeter of said loop to vary the inductance of said loop, connections for applying high frequency electrical oscillations to said loop.

WILLIAM H. ANDERSON. ALBERT C. FREY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 928,371 Fessenden July 20. 1909 Number Name Date 1,909,610 Carter May 16, 1933 1,911,980 Vance May 30, 1933 2,092,069 Hollman Sept. 7, 1937 2,247,212 Trevor June 24, 1941 2,308,043 Bierwirth Jan. 12, 1943 2,367,576 Harvey et al Jan. 16, 1945 2,396,004 Gilbert Mar. 5, 1946 2,414,280 Thomas Jan. 14, 1947 2,438,477 Dodds et al Mar. 23, 1948 2,464,404 Gillespie Mar. 15, 1949 FOREIGN PATENTS Number Country Date a 226,586 Great Britain Dec. 29, 1924 234,331 Great Britain May 28, 1925 OTHER REFERENCES Publication: Scott, The Role of Frequency in Industrial Dielectric Heating," Transactions, Electrical Engineering, August 1945, pages 558- 562. 

